Currently high-speed data signals are routed on printed circuit boards using a strip line configuration. The strip lines help to reduce electromagnetic emissions and cross talk. High-speed data transmitted along the strip lines located either on the surface or embedded in the PCB must be passed to and from a host. This is usually done with an electrical connector into which the PCB is plugged. Electrical connection is made via strip lines located on the surface of the PCB. However, this causes discontinuities in the data line impedance, which has a negative effect on the data signal causing quality issues.
As can be seen in FIG. 1, which shows a cross section of PCB 10, differential strip lines 15 and 16 are embedded within the PCB and ground planes 11 and 12 are disposed on the upper and lower surfaces. FIG. 2 shows a longitudinal cross section of the PCB shown in FIG. 1. The strip line 15 is connected to an electrical component 17, such as an integrated circuit (IC), disposed on the surface of the PCB by means of via 21 and a further strip 8. The PCB is plugged into electrical connector 20 and electrical connection is established to the IC through a second via 22 and yet a further strip line 9. As can be seen, both strip lines 8 and 9 are located on the surface of the PCB. The discontinuities as indicated at points “A” along the strip lines and vias (8, 21, 15, 22, and 9) affect the signal quality due to changes in the impedance characteristics. The characteristic impedance of a transmission line such as a differential pair will change according to its distance from a ground plane, dielectric impedance and distance from another transmission line. All these parameters are affected by discontinuities.
Thus it is an object of the present invention to remove as many of these discontinuities as possible while maintaining low levels of electromagnetic emissions and increasing the data reliability. It is a further object of the present invention to provide a PCB and connector design which is compatible with pluggable optical transceivers.